Many applications are known wherein a liquid is reacted with a particulated solid (“solids”) to enhance one or the other of the liquid or the solids (or both) for commercial benefit. One common application is to react a liquid “lixivant” with a solid to extract a soluble compound from the solid by way of percolating or washing the solid with the liquid. (Accordingly, a “lixivant” is a liquid used for this purpose.) This process is commonly described as “leaching”. Typical examples include extracting valuable metals from ores containing the metals by contacting the ores with a lixivant. The extracted metals will then be in solution with the lixivant, and can be later removed from the lixivant by known chemical processes, such as chemical precipitation, to render a relatively pure form of the extracted metals, or a form that can be subsequently processed to render a relatively pure form of the extracted metals. One example is to wash ore containing gold with a lixivant containing cyanide to remove the gold from the ore. Other examples include washing oil shales with a solvent to extract petroleum from the shales, and washing coal with a sulfur-extracting liquid to remove sulfur from the coal. Yet another example includes contacting contaminated soil with a liquid-borne biological agent (or agents) to thereby decontaminate the soil.
In all of these processes the volumes of solids to be treated are typically considerable—on the order of tens to thousands of metric tons per day. In the case of ore leaching (to remove valuable metals from ores containing the metals), the most common process is to pile the ore into a “heap” on a leach pad, and then to introduce a lixivant onto the top of the heap. After the lixivant has passed through the ore heap via gravity, the lixivant is collected and processed to remove the extracted metals from the lixivant. The spent ore is then discarded (as for example by moving it to a spent ore pile), and new unprocessed ore is then placed on the leach pad, and the process repeated. Such leach pads often occupy areas covering many acres, and in some cases square miles. Due to the nature of the lixivants used, and the metals being extracted from the ores, leach pads are typically subject to significant environmental controls to reduce the possibility of potential contamination of soil surrounding the leach pad. Further, the ore leaching process via ore heaps and leach pads is a slow process. Common leach times (i.e., the time between when the ore heap is initially formed and the lixivant added to the ore heap, and the time when the ore is considered “spent” and is removed from the leach pad) are on the order of months. A six month leach time is not uncommon.
Other prior leaching methods and apparatus include: (1) batch tank leaching, (2) agitated vat leaching, (3) counter-current tank leaching, (4) permanent pad heap leaching (described briefly above), (5) re-usable pad heap leaching, and (6) bio-heap leaching. A common description for each of these methods and apparatus is a “leach circuit”.
The specific shortcomings of the prior art are as follows.
For agitated vat leaching, the basic operational concept is to provide an elevated contact rate of lixivant and other additives to the surfaces of the ore particles by (a) increasing the surfaces of the ore which can be accessed by the lixivant by grinding the ore to a particle size that exposes the desired metal or mineral value, (b) vigorously agitating the ore and lixivant so as to provide an elevated level of contact between unconsumed reaction agents, and (c) to readily remove reaction outputs so as to maintain in majority concentration the unconsumed reaction agents.
The shortcomings of such a process include: (1) significant capital and operational costs are associated with grinding the ore to a small particle size and vigorously agitating such a dense media as an ore slurry; (2) the processing time required for the desired recovery level—as short as 24 hours in the typical case—in conjunction with the size limitations for a vessel which will afford reasonably good economic access of the agitation mechanical to the ore slurry, necessitates a large number of containment vessels, which in turn necessitates a plant of commensurate size to contain and support the operation of the containment vessels, all of which requires significant capital and real estate to construct; (3) small particle sizes typically present challenges for disposal of spent ore since special impoundments are typically required to de-water and stabilize it as permanent fill; (4) because of the relatively high capital and operating costs of such a leach process, the method is not economical for very low grade ores or ores which require leach times in excess of 24 hours to achieve economic recovery; (5) batch processing contains an inherent limitation in that there is wasted economic time between batch operations; and (6) because of the complexity of such a mechanically intensive process, design and construction times for the plant are relatively long (as compared to heap leaching, for example).
Heap leaching is an alternative to vat leaching and attempts to address the limitations of vat leaching with respect to low grade ores and ores that require longer leach recovery times (e.g., using certain oxides and certain sulfides). The basic operational concept of heap leaching is to trade-off leach recovery time for leach circuit processing size or volume by (1) secondary or tertiary crushing of the ore instead of crushing to an ultra-fine grain size, (2) agglomerating the ore into relatively uniform ore spheres to increase permeability of lixivant and increase contact effectiveness rather than agitating the ore, (3) stacking in broad, relatively shallow piles on an impermeable layer instead of batching in expensive vessels, (4) sprinkling lixivant on the ore, letting it trickle down under the action of gravity alone through the ore, and collecting the pregnant solution from perforated pipes on the bottom of the heap rather than submerging the ore within a vat or tank, (5) blowing air into the heap (as in the case of bio-heap leaching), and (6) removing the ore continuously from the pad as in the case of re-usable pads to make heap leaching a more continuous rather than a batch process.
Although heap leaching extends leaching technology to lower grade and harder-to-leach ores that are not economically done with vat leaching because of the implied processing volume required, heap leaching is less effective in extracting metals and the like from the ores, primarily due to the absence of submersion of the ore in the lixivant and agitation of the ore (as in agitated vat leaching). Of particular concern in the use of a trickle-type application of lixivant to a stack or pile of ore on a leach pad is channeling of the lixivant, leaving significant portions of the leach pile without sufficient lixivant to extract the theoretical maximum recoverable metals using the heap.
Another inherent shortcoming of heap leach is the inability to control environmental inputs such as temperature and oxygenation of the heap, which are critical factors in bio-heap leaching where the effectiveness of the bacteria is closely dependent on these variables.
Perhaps the greatest shortcoming of heap leaching is the capital and operating costs associated with large volumes of material, especially in the case of re-usable pads. Whereas in vat leaching the ore is transported in a slurry in pipe conduits, heap leaching, because of the large geometric extents of leach pads and complexity of stacking a stable heap, has been performed almost exclusively with conventional overland conveyors and specialized spreading and reclaim conveyors, which imply high capital and operating costs as compared to the compact plant piping of vat leaching.
What is needed then is an economical, efficient method and/or apparatus to react solids and liquids with one another that achieves the benefits to be derived from similar prior art apparatus and methods, but which avoid the shortcomings and detriments individually associated therewith.